The present invention relates to a phased array antenna having digital phase shifters and, more particularly, to a phased array antenna with a function of compensating for changes in characteristics ascribable to temperature.
A phased array antenna is capable of scanning a beam electrically, and is used in a microwave landing system (MLS), for example. In MLS, a phased array antenna located on the ground transmits a reciprocating beam to aircraft, while the aircraft measures the interval between a pair of received beams and thereby determines the azimuth and elevation angle thereof. This allows the aircraft to land along a predetermined route. A phase array antenna for the MLS application is generally required to have an accuracy of the order of 1/100 degrees as to beam angle or scanning angle. In practice, however, the characteristics of various components of the antenna, such as a power divider for distributing power to individual antenna elements, are susceptible to temperature since the system itself is situated outdoors. Hence, not only the beam pointing but also the beam shape or the side lobe level are changed and cannot meet the accuracy requirement unless compensation is effected.
In light of this, it has been customary to provide the antenna with an air conditioner. Although the air conditioner is applied for maintaining the temperature around the antenna constant and, therefore, for suppressing the changes in characteristics ascribable to temperature, it brings about various problems, such as the increase in running cost and low reliability.
The use of a monitor manifold associated with a phased array antenna is a conventional approach to reduce the change in beam pointing due to temperature, as disclosed in, for example, U.S. Pat. No. 4,536,766 entitled "SCANNING ANTENNA WITH AUTOMATIC BEAM STABILIZATION" (Aug. 20, 1985). Specifically, while the monitor manifold detects a scanning angle, the scanning timing is changed on the basis of the resultant error. This kind of approach, however, simply corrects the scanning angle by changing the scanning timing. It cannot compensate for the changes in beam shape and side lobe level. As a result, with such a scheme, it is not practicable to prevent the MLS performance from being degraded by the changes in beam shape and side lobe level.